From: Daniel Sheppard <daniels@...>
Date: 2005-10-21T14:47:04+09:00
Subject: Re: Reflection, observ(er|able) instance vars?

Problem with the == method you defined is that hash equality != object
equality - especially for your hashing method.

This module will provide you with the hash and eql? methods (alias those
to other methods as appropriate). The hash function is cached, and reset
whenever a var= method is called (make sure you don't go modifying
hashed vars directly). 

This is probably not the only/best way to do this, but it made sense to
me:

module FastHashEquals
    def attr_hashing( *arr )
        attr_reader *arr
        hashers = class_eval "@@__hashers ||= []"
        hashers.concat(arr)
        arr.each do |var|
            define_method(var) {
                instance_variable_get("@#{var}")
            }
            define_method("#{var}=") {|obj|
                instance_variable_set("@#{var}", obj)
                instance_variable_set(:@__equals_hash, nil)
            }
            define_method('eql?') { |other|
                hash === other.hash && hashers.all? { |x|
                    send(x) == other.send(x)
                }                
            }
            define_method('hash') {
                @__equals_hash ||= hashers.inject(0) { |s,x|
                    s + instance_variable_get("@#{x}").hash
                }              
            }
        end
    end
end

To use:

class HashTest
	extend FastHashEquals
	attr_hashing :one, :two, :three
end

> -----Original Message-----
> From: Trans [mailto:transfire@gmail.com] 
> Sent: Friday, 21 October 2005 9:32 AM
> To: ruby-talk ML
> Subject: Re: Reflection, observ(er|able) instance vars?
> 
> Use Module, or better yet your own mixin. And just do the 
> class eval yourself w/o metaid, it's pretty easy
> 
>     def ==( other )
>       hash == other.hash
>     end
>   end 

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